1. Field of Invention
This invention relates to savings up to 50% or more of shielding gas in the MIG welding process while improving weld start quality.
Gas metal arc welding (GMAW) is commonly referred to as metal inert gas welding (MIG). The term MIG welding is used in this document. In the MIG welding process molten metal is produced by an electric arc. A welding wire is fed into the arc zone by a feeding mechanism. A suitable power source is connected between the workpiece to be welded and to the welding wire passing though a welding torch. Welding power, welding wire, and shielding gas are usually transported through the torch. The welding torch is usually attached to a flexible cable assemble and is manipulated by the welding operator. Molten metal comprising the weld is derived from the materials to be welded and the welding wire. The molten weld metal is protected from the surrounding air by a shielding gas. The welding wire is melted into droplets most of which are transported to the weld puddle and solidify into the weld. However about 2 % to 4% of the droplets are expelled from the weld zone and are referred to as weld spatter.
The shielding gas employed to protect the molten metal formed by the electric arc can be a number of gases such as argon, carbon dioxide, and helium. Mixtures of these and small amounts of other gases are employed to provide the desired welding performance. Shielding gas is supplied from a pipeline or a high-pressure cylinder. Fabricating shops with a large number of MIG welders often have the shielding gas distributed to the welding area through a pipeline from a centrally located gas source. From the pipeline it is usually delivered to each welding machine though a flexible shielding gas delivery hose. In an application such as a shipyard, the shielding gas delivery hose may be in excess of 30 m (100 feet) long. Pipeline pressures are often set at 345 kilopascal, kPa (50 pounds per square inch, psi) or higher. When high pressure cylinders are employed for the gas supply a regulator is used with a preset output of 172 kPa (25 psi, pounds per square inch), 207 kPa (30 psi), or in some common regulators 345 kPa (50 psi). The output pressure level of the regulator is dependent on the manufacturer and model. For MIG systems using carbon dioxide as a shielding gas supplied in cylinders, it is common to employ a regulator with 550 kPa (80 psi) output. This higher outlet pressure reduces the possible formation of ice crystals in the regulator/flow-control system as the carbon dioxide gas pressure is reduced from that in the cylinder. For each welding system a variable flow-control valve or suitable flow-control device is usually located at the pipeline supply or incorporated immediately after the regulator. This flow-control device allows adjustment of the shielding gas flow to the appropriate rate needed for welding. A flow measurement gauge may be included as part of the flow control system.
It is most common for the hose used to deliver the shielding gas from the flow-control device at the gas pipeline or cylinder to the welding machine or wire-feeding device to be 6.4 mm (¼ inch) in internal diameter. This hose may be longer than 30 m (100 feet) for some applications. To turn the flow of shielding gas on and off in commercial MIG welding systems, it is common to employ an electrically operated gas solenoid in the wire feeder or welding machine. When welding is started, usually by means of an electrical switch on the welding torch, the gas solenoid is opened allowing shielding gas to flow through the welding torch to the weld zone. The electrical switch may simultaneously engage the wire feed mechanism and power source.
In most systems, the flow of shielding gas is controlled by a flow-control device adjusted to achieve the desired shielding gas flow. It is common for this flow to be set from 9.4 Liters per minute, L/min (20 cubic feet per hour, CFH) to 19 L/min (40 CFH). Gas flows rates much in excess of this level can cause turbulence in the shielding gas stream as it exits the welding torch. This turbulence allows the surrounding air to be aspirated into the gas-shielding stream, degrading weld performance. In many systems, the gas pressure needed at the gas solenoid to provide the proper flow of shielding gas is less than 35 to 70 kPa (5 to 10 psi). This pressure requirement will vary somewhat as welding is performed and restrictions occur in the welding torch cable as it is bent and twisted during operation. The pressure requirements will also increase as weld spatter builds up in the welding torch shielding gas nozzle and gas passages in the front portion of the torch. If properly designed the gas supply system will automatically compensate for these restrictions and maintain the shielding gas flow at the preset value.
While welding, the gas solenoid valve is open, and the pressure in the gas delivery hose is only that needed to establish and maintain the desired flow. The flow-control device located at the gas source is set for the desired shielding gas flow rate and indirectly establishes this pressure. The system may incorporate a flow-measuring gauge. Portable flow-control gauges are also available. The portable gauge is put over the end of the torch shielding gas nozzle with the wire feed mechanism temporarily disengaged. While reading the gas flow on the gauge, the proper shielding gas flow is set by adjusting the flow-control device. When welding commences the pressure in the gas delivery hose near the solenoid is typically less than 35 to 70 kPa (5 to 10 psi) depending on the torch type, length, and plumbing restrictions. When welding is stopped the solenoid closes and flow of shielding gas from the solenoid to the torch stops. However, the gas flow continues to flow though typical flow control devices employed and fills the gas delivery hose until the gas pressure in the hose reaches the pressure in the pipeline or that pressure set by the regulator if present. The pressure in the gas delivery hose than rises from what was needed to establish the proper flow level to the outlet pressure in the pipeline or that set by the regulator. The excess pressure stores shielding gas in the gas delivery hose connecting the flow-control device to the welding machine or wire feeder until the solenoid is opened at the start of the next weld. When welding is restarted, this excess shielding gas is expelled very rapidly, often within less than about 1 to 3 seconds.
Depending on the number of starts and stops versus the overall welding time, wasted shielding gas can exceed 50% of the total gas usage. An article in the June 2000 Fabricator Magazine entitled “Shielding Gas Consumption Efficiency,” page 27, col. 2 & 3 sites the fact that most shops are using from about two to over ten times more gas than is possible. In the same article a significant waste is described (page 29, Col. 3 & 4) as attributable to the storage of excess shielding gas in a commonly employed 6.4 mm (¼ inch) inside diameter shielding gas delivery hose. A more recent article was published in the January 2003 issue of Trailer Body Builders Magazine entitled “How to Save 20% on Welding Costs.” On page 46 col. 3 of this article a representative from a leading manufacturer of shielding gases is quoted as saying, “A minimum of 142 Liters, L (five cubic feet) of gas is required to weld 0.45 kg (one pound) of wire, but the industry average usage is 850 L (30 cubic feet).” Since it is very unusual to need more than 225 to 280 L (8 to 10 cubic feet) of shielding gas per 0.45 kg (pound) of wire this statement means the average user consumes from three to six times the amount of shielding gas theoretically needed. Using the lower 3 times usage value and estimating the average retail price and annual volume; American consumers are wasting over 500 million dollars annually in shielding gas employed for MIG welding. Very high shielding gas surge flow rates at the weld start can momentarily reach in excess of 95 L/min (200 CFH). This flow rate is much higher desirable for good weld start quality. Weld start quality is degraded because excess shielding gas flow rate creates air aspiration in the shielding gas stream. Bernard, in U.S. Pat. No. 3,275,796 (1966) col. 5, 46 to col. 5, 49 states; “The system described provides a purging of the weld area where the arc is to be established by a strong blast of shielding gas to remove rust, dirt, and slag particles before the arc is established.” In tests it was found that the excess shielding gas flow rate at the start is inconsequential in performing the tasks defined by Bernard. In addition the excess flow rate and resulting turbulence in the shielding gas stream can create internal weld start porosity and excess spatter.
2. Description of Prior Art
There have been devices sold which provide solutions to the problem of excess gas surge at the weld start:    (a) One device designed to reduce shielding gas loss at the weld start is described by Stauffer in U.S. Pat. No. 4,341,237 (1982). This device is of complex construction involving several mechanical elements and a surge tank to store and control this excess shielding gas. The numerous internal connections create the potential for leaks. The device incorporates a low pressure regulator to reduce gas waste and the surge storage tank is placed after the low pressure regulator. The surge tank designed to provide additional shielding gas at the weld start is large in size. Gas storage and extra gas supply at the weld start can only exist if the gas pressure in the storage device is higher than that needed while welding. The extra gas available for the weld start deliverable from this storage device must therefore rely on a regulator pressure which is higher than that needed to supply the desired flow of shielding gas while welding. If the device is to be effective in reducing shielding gas waste the pressure set by the regulator must be substantially lower than that in the incoming gas delivery line as is stated in the patent. Thus, the surge tank in the device described must be large since the extra gas available is proportional to the pressure differential between the pressure set at the regulator and that pressure needed to create the desired gas flow while welding. The practical implementation of a device labeled as being built under this patent contains a storage device which is relatively large.
In addition, if the shielding gas pressure is set at a substantially lower pressure than the gas delivery line there is little extra pressure available to compensate for restrictions which occur in the welding torch cable due to twisting and bending while welding. There is also little extra pressure to compensate and maintain the preset flow when weld spatter accumulates in the torch gas nozzle and/or blocks the torch gas passages at the nozzle end. Measurements made with a device employing this type of low pressure regulator design showed preset gas flow reduced about 20% when simulating spatter blockage in the weld torch. This is one of the reasons gas delivery systems were designed to operate at higher pressures. These higher pressure systems employ the principle of gas flowing though a critical orifice reaching a limiting velocity based on the orifice size and the pressure upstream of the orifice. The pressure downstream of the orifice will have little or no influence on the flow rate as long as that pressure is less than about one half of the upstream pressure. A gas delivery system designed to utilize higher pressure is referred to as a self compensating system.
Some simple low pressure devices have been used to reduce gas waste. However some of these devices provide no extra shielding gas at the weld start causing porosity due to air that enters the torch shielding gas cup, torch body, and torch cable when welding is stopped. Also the lack of extra gas pressure at the solenoid does not provide compensation for restrictions that occur in the torch body due to spatter build-up causing variations in shielding gas flow while welding.    (b) Another method occasionally used to reduce gas surge upon the initiation of welding is to incorporate a flow-control orifice at the solenoid end of the shielding gas delivery hose. This device is sometimes sold with the intent to reduce gas waste. The device can give the perception that gas waste is significantly reduced since the momentary high gas flow surge at the start of welding is reduced, however the gas waste may still occur. The orifice size is often selected to restrict gas flow at the weld start but not steady state gas flow while welding. Depending on the delivery pressure of the regulator these devices can employ very small orifices, as small as about 0.8 mm (0.032 inches). However, the orifice size is usually set to control the transient weld start gas flow rate well above that desired for steady state welding. This is necessary since differing welding torches, torch lengths, and internal plumbing restrictions require differing pressures at the solenoid to obtain the desired gas flow through the torch. Also it is often desirable to allow the shielding gas flow rate to be adjusted by the welding operator during production to compensate for drafts in the weld area or the type of weld joint. When the orifice size selected is larger than needed to control the shielding gas flow while welding, gas pressure in the shielding gas delivery hose at the solenoid valve end while welding reduces to that needed to obtain the desired flow.
Some torches and systems require about 35 kPa (5 psi) gas pressure at the solenoid to provide the desired flow. This is significantly lower than the pipeline or regulator fixed output pressure. When welding is stopped the gas solenoid closes and the pressure in the shielding gas delivery hose increases to the pipeline supply pressure. This pressure is often 345 kPa (50 psi) or higher or the delivery pressure of the regulator, i.e. 172 kPa (25 psi), 207 kPa (30 psi), 345 kPa (50 psi), or 550 kPa (80 psi). Once welding commences after several seconds, the flow rate reduces to the lower level set at the flow-control device near the gas source. Therefore, the pressure in the welding gas delivery hose near the solenoid end reduces to the level needed to achieve the desired flow, perhaps 35 kPa (5 psi). Experiments show, once the solenoid valve is open at the start of welding, even when a typical flow-restriction orifice is used in the system, a majority of the excess gas stored in the shielding gas delivery hose passes through the torch in about 3 to 5 seconds or less. Therefore, if the weld occurs for more than about 3 to 5 seconds in time, a similar amount of excess shielding gas is lost as if the restriction orifice was not present. The loss takes longer to occur, perhaps about 3 to 5 seconds, but it occurs. In addition, if the orifice were sized to restrict flow to exactly that needed for the desired flow there would be no extra shielding gas available at the start. It is desirable to have some extra shielding gas at the weld start to purge the torch system of air. Air will enter into the torch gas nozzle, torch body and torch cable when welding is stopped.
The phenomenon of a surge flow restrictor not significantly reducing shielding gas waste is not well understood. The reduction in gas surge the restrictor provides gives the perception that the waste is eliminated or significantly reduced. Hanby in U.S. Pat. No. 6,390,134, 2002 defines the use of restrictors to control shielding gas flow. Hanby states, col. 1, 33, “Gas-surge wastes valuable inert welding gas.” Hanby subsequently states in col. 2, 7 to col. 2, 9, “Gas-flow may be set to any level below the maximum threshold by adjusting the flowmeter, just as in normal welding operation.” This gives the impression that a significant savings in gas waste will exist with this approach or use of the claimed product, which it will not! Uttrachi in U.S. Pat. No. 6,610,957 (2003) defines the situation properly, namely col. 3, 37 to col. 3, 40; “Orifice restriction devices help reduce high flow gas surge at the weld start and the resulting degradation of the weld but often do not eliminate or significantly reduce shielding gas waste.”    (c) Another method of reducing shielding gas waste is obtained by decreasing the volume of shielding gas stored in the delivery hose. Assuming a given length hose is needed to achieve the desired welding machine configuration, the other dimension controlling the volume in the shielding gas delivery hose is the internal cross sectional area. Uttrachi in U.S. Pat. No. 6,610,957 (2003) defines the use of a small inside diameter shielding gas hose with the addition of a surge restricting orifice at the gas solenoid end of the shielding gas delivery hose to limit gas surge at the weld start to improve the quality of weld starts. This system reduces shielding gas waste, but may have excess pressure drop for very long shielding gas delivery hose lengths. This excess pressure drop may limit the gas flow attainable below desired levels.